In the coatings art, there has been an increasing emphasis upon lowering the volatile organic content of paints and similar coatings compositions. This concern stems both from the increasing costs of organic solvents as well as problems associated with the environmental impact of the use of such volatile organic solvents and the costs involved in ameliorating such environmental impact. To address these concerns, research efforts in the coatings art have recently been focused upon water-borne coating compositions. To achieve dispersibility in an essentially aqueous medium, polymer resins employed as the principal binder resin and as pigment grind resins for water-borne coating systems must be stabilized. The mode of stabilization may be ionic or nonionic. Typical ionically stabilized water-borne paint formulations employ polymers which contain a fraction of ionizable functional groups such as carboxyl group-containing monomers. Neutralization of the acid functionality provides the degree of water miscibility required to stabilize the resin in aqueous systems.
Typical nonionically stabilized water-borne resins employ monomers which contain a certain fraction of nonionic hydrophilic groups for stabilization. Water-dispersible nonionically stabilized polyurethane resins useful in the preparation of coating compositions are well known in the art. These compositions typically contain hydrophilic centers which impart water miscibility to otherwise hydrophobic polymers. The most effective hydrophilic groups are polyether or poly(oxyalkylene) groups.
Water-dispersible nonionic polyurethane resins known in the prior art are best exemplified by those disclosed and claimed in U.S. Pat. Nos. 4,190,566 to Noll and 4,794,147 to Savino, et al. In both patents, the polyurethane resins are the reaction products of a polyester prepolymer, diols, triols, a polyether alcohol, and a polyisocyanate. In these resins the polyether segments, which serve to stabilize the resin in aqueous medium, are located within the polyurethane segments of the polymer. The polar, water miscible polyether segments are thus located at some distance in the polymer backbone from the more hydrophobic polyester segments. This arrangement does not make the best use of the stabilizing effect of the polar, hydrophilic polyether segments with the result that a higher fraction of polyether segments is required in the final polyester polyurethane resin to achieve stabilization in aqueous medium.